This invention relates to electrical machines, and is concerned more particularly, but not exclusively, with electrical motors. The invention is particularly applicable to electrical fans and pumps.
Electrical motors are commonly used for powering fans and pumps. In an application such as a hand dryer, it is particularly important that the motor should be capable of driving the fan in a power efficient manner which enables the fan to initially be driven at low speed and to then be rapidly accelerated to a high speed so that hand drying can be effected in the shortest possible time. However, this imposes particular requirements on the motor for driving the fan since the power required to drive the fan increases in proportion to the cube of the speed. Furthermore it is a requirement for such an application that the motor should be capable of driving the fan at high speed without producing excessive acoustic noise.
It is an object of the invention to provide an electrical machine exhibiting high power efficiency and low acoustic noise.
According to the present invention there is provided an electrical machine comprising a rotor without windings, a stator having a winding, and circuit means for controlling the current in the winding during rotation of the rotor, characterised in that the circuit means includes a capacitance bridge defining a variable voltage point to which one end of the winding is coupled to form a resonance circuit, and switch means coupled to the winding and switchable in synchronism with rotation of the rotor to produce cyclically varying current pulses which are caused to decay naturally by the build-up of voltage at the variable voltage point of the capacitance bridge during each cycle of excitation.
The provision of a resonance circuit, incorporating the winding and the capacitance bridge defining the variable voltage point at which the voltage varies substantially (by at least 50% of the average midpoint voltage) during each cycle of excitation, ensures that the current in the winding can return naturally to zero at the end of each excitation pulse, thus providing more efficient power operation and reducing the acoustic noise of the machine. Such reduction in the acoustic noise level is caused by a decrease in the sharp voltage transients across the winding, such transients being one of the dominant causes of acoustic noise.